Coating undesirably pure monocalcium phosphate for heat treatment



Patented Feb. 3, 1942 COATING UNDESIRABLY PURE MONOCAL- OIUM PHOSPHATE FOR HEAT TREATMENT William H. Knox, Jr., Nashville, Tenn., assignor to Victor Chemical Illinois Works, a corporation 01' i No Drawing. Application May 15, 1939,

Serial No. 273,809 v 4 Claims.

This invention relates to a method of coating undesirably pure monocalcium phosphate for heat treatment and the product thereof.

' In my co-pending application, Serial No. 226,- 180, filed August 22, 1938, of which this application is a continuation-in-part, is described a new crystalline anhydrous monocalcium phosphate The product therein described is desirably made from a phosphoric acid containing impurities characteristic of blast furnace phosphoric acid. The presence of such impurities produced, upon proper heat treatment, as described in the patent of Julian R. Schlaeger, No. 2,160,232, issued May 30, 1939, a substantially pyrophosphate-free anhydrous monocalcium phosphate having a thin, autogenous, glassy, substantially complete, relatively insoluble coating. r

The present invention relates to the preparation of monocalcium phosphate, preferably anhydrous from the beginning, in which for one reason or another it is desired to produce initially an undesirably pure product. suchla pure monocalcium phosphate will not heat treat properly underordinary circumstances to produce the improved baking acid described in the Schlaeger patent. By means of the present invention,

however, undesirably pure materials may be superficially supplied with a thin layer of coatlug-forming impurities. Once these impurities have been properly applied in such a superficial layer, heat treatment in accordance with the process of the Schlaeger patent will produceanhydrous monocalcium phosphate having thereon a glassy, autogenous, thin, substantially complete and relatively insoluble coating.

The present process may be employed in those instances where undesirably pure monocalcium phosphate is available, or it may be desirable to' produce initially undesirably pure monocalcium phosphate in order to'eliminate' deleterious impurities. For example, magnesium appears to be an undesirable ingredient of the monocalcium phosphate an d yet it is always found in the lime of commerce which is the basis of the calcium content of the monocalcium phosphate. There- .fore, it may be desirable in some cases to prepare anhydrous monocalcium phosphate by fractional crystallization thereof from a liquor. The resulting product by itself is undesirable for incorporation in a baking preparation because of its instability against hydration. Likewise, it will not properly heat treat. It may, however, be processed by the addition of a superficial layer of coating-forming impurities, particularly alkali metal salts, such as the salts of potassium.

Similarly, the anhydrous monocalcium phosphate of my Patent 2,160,700, issued May 30, 1939, may be produced in undesirably pure form and supplied with a superficial layer of impuri- 5 ties.-= The process is also applicable to the treatment, of dehydrated monocalcium phosphate.

It is sometimes possible to eliminate deleterious impurities in the formation of the monocalcium phosphate where it would be extremely difflcult or expensive to removethese impurities from the initial reagents.

The impurities may be applied to the surfaces of the monocalcium phosphate particles by any means which will insure, upon heat treatment, a

used and under the conditions employed will not,

yield water in substantial quantities to the anhydrous monocalcium phosphate. For example, a very concentrated aqueous solution, such as concentrated phosphoric acid, would not be considered hydroscopic, nor would a concentrated aqueous solution of other materials be so considered where employed at sufliciently high temperatures such that no absorption of water by the anhydrous monocalcium phosphate occurs.

Preferably, however, non-aqueous solvents such as alcohols, for example ethyl, methyl or butyl, acetone, formic acid or the like, may be employed. For example, an alcoholic solution of potassium hydroxide may be applied to anhydorus monocalcium phosphate particles, and the alcohol evaporated. This leaves a coating of 40 potassium hydroxide upon the surface of the particles,-which,, upon heating at the proper temperatures, reacts with the monocalcium phosphate to form a phosphatic skin coating of the desirable glassy, substantially insoluble type. It is also-possible to dust the particles with easily fusible alkali metal compounds, provided the particles are sufllciently manipulated so that substantially complete coatings will be formedupon heating. As an example of the invention, solid crystalline anhydrous monocalcium phosphate was produced by interact-ion of pure phosphoric acid produced by burning commercially elemental phosphorus and C. P. lime- The anhydrous 5 5 monocalcium phosphate so produced showed only moderate improvement in reaction rate at heat treatment at 205 C. for a period of one hour.

*To this material potassium hydroxide in ethyl alcohol solution was applied and the alcohol evaporated. The following Table I illustrates the results accomplished with various amounts of potassium hydroxide, the coated pure product being heat treated in each case for one hour at 205 C.:

Table I Amt. oi K added Item None 0.2% 0.4% 0.6% 1.0%

Neutralizing value 88.0 83. 0 81. 6 78.4 81. 4 Pyrophosphate contcnt 0.8 1.6 1.4 2. 4 2. 0 Free acid content 0.0 0. 0 0. 0 0. 8 Primary reaction rate 1 cubic centimeters 100 81 72 72 N Secondary reaction rate do 56 66 74 73 phosphate was provided with a superficial layer of potassium hydroxide by moistening the surfaces of the crystals with. potassium hydroxide solution in ethyl alcohol and evaporating the alcohol at about 0. Various amounts of potassium hydroxide were applied, and then the superficially coated particles were treated with 64.8" B., HaPOi dissolved in 95% ethyl 9.1--

cohol to convert the coatings respectively to K2HPO4, KH2PO4 and KH2PO4.H3PO4. The alcohol was then evaporated and the resulting coated product heat treated for one hour at 200 C; The results of these examples are shown in the following Table II.

Table II Baking powder a-fig Neufyrophosreaction rates Calculated compn. added, tralizphate conof coating 'g; ing tent, perk value cent Pri- Secondmary ary Cc. Cc. K HP O4 0.2 88.2 74 72 0. 4 83. 8 1 64 76 0.6 83.6 1. 7 39 97 KHzI O4 0. 2 85. 8 80 64 0. 4 85. 6 56 87 0:6 82. 8 1. 7 24 111 KHZP O .H;P O 0. 2 87. 0 79 71 0. 4 85. 4- 50 95 0. 6 84. 0 1. 7 22 116 Similar results were obtained in another series of tests where the heat treatment was conducted 7 at 210 C.

, a volatile non-aqueous solvent.

The superficial coatings may be applied by intentionally providing a slight amount of excess free acid in the anhydrous monocalcium phosphate and then neutralizing this free acid with a basic substance such as potassium carbonate. The use of such a method, however, requires great care in order to avoid the presence of excess quantities of fluxing agents which may tend to produce pyrophosphate.

The desired impurities may likewise be added in water solution during the formation of anhydrous monocalcium phosphate as described in my Patent 2,160,700, wherein water is sprayed upon the hot reaction mass in order to control its temperature. This water may be a. solution of the desired impurities in the desired amount.

Obviously the herein described processes may be employed to add either all the impurities required, or part of the impurities may be added in other ways, or may be present in the initial materials.

What I claim as new, and desire to secure by Letters Patent, is:

1. In the formation of anhydrous heat-treated monocalcium phosphate having a thin, glassy, substantially complete, relatively insoluble coating, the steps of adding to undesirably pure crystalline monocalcium phosphate a thin superficial film of a soluble potassium phosphate compound and then heating the'monocalcium phosphate for a prolonged period at a temperature above C. and below a temperature at which substantial conversion to molecularly dehydrated phosphate occurs to form a thin, glassy, substantially complete, containing less than 3% of pyrophosphate and relatively insoluble coating surrounding an anhydrous monocalcium phos-, phate core.

2. In the formation of anhydrous heat-treated monocalcium phosphate having a thin, glassy, substantially complete, relatively insoluble coating, the steps of adding to undesirably pure crystalline nhydrous monocalcium phosphate a thin superficial layer of a compound of the class consisting of potassium hydroxide, potassium carbonate and potassium phosphate and then heating the anhydrous monocalcium phosphate for a prolonged period ata temperature above 140 C. and below a temperature at which substantial conversion to molecularly dehydrated phosphate occurs to form a thin, glassy, substantially complete, relatively insoluble coating, the heat-treated composition containing not substantially more than 3% of pyrophosphate.

3. The method as set forth in claim 2, in which the potassium compound is the hydroxide.

4. The method as set forth in claim 2, in which the potassium compound is the hydroxide, and in which the potassium compound is applied in WILLIAM .H. KNOX, JR. 

